Stop pretending this has anything to do with artistic expression, any kind of need to expand the stories of these characters or anything other than base corporate greed emanating from a market that has met its full saturation point (and then some).
That could pretty much describe most of DC and Marvel's output. Any artistic expression has always been in the service of making money for the corporation. That's just the way it is, that's their business, no art for art's sake. An Idea may or may not have much artistic merit, but if it has the potential to sell well, then you have a corporate comic.
Watchmen sequels are just a little more obvious because it was a limited series to begin with, and it's always been a sacred cow.